Digestion and absorption in the upper GIT Flashcards
What are the principle site of carbohydrate digestion?
1) Mouth
2) Intestinal lumen
What are the enzymes that are involved in the digestion of carbohydrates?
Glycosidase (specific to the bond they are breaking)
- Final product being monosaccharides
What is the enzyme that synthesizes glycosidic bonds?
Glycosyltransferase
What is the carbohydrate bond that is broken down by salivary amylase?
a-1,4 glycosidic bond
What are the different outcomes of salivary alpha amylase digestion?
1) Dextrin (branched and unbranched oligosaccharides)
2) Disaccharides (some are resistant to amylase)
- dISACCHARIDES NEED DISACCHARIDASE ENZYME TO HYDROLYZE THEM INTO MONOSACCHARIDES
What is the role of the pancreatic amylase?
- Once the salivary alpha-amylase reaches the stomach they get denatured
- The pancreatic amylase continues the process of starch digestion by breaking the a-1,4 linkages
Where does the final carbohydrate digestion occur in the body?
At the intestinal brush border
What type of carbohydrate bond is broken down by isomaltase?
- a-1,6
- Its products are 2 glucose molecules
What type of carbohydrate bond is broken down by maltase?
- a1,4
- Its products are 2 glucose molecules
What type of carbohydrate linkage is broken down by Lactase?
- B-1,4
- Its products are glucose and galactose
What type of carbohydrate bond is broken down by sucrase?
- a1,2
- Its products are glucose and fructose
What type of carbohydrate bond is broken down by trehalase?
- a-1,1
- Its products are 2 glucose molecules
Where does the absorption of monosaccharides occur?
In the duodenum and upper jejunum
What is the transporter of glucose and galactose?
- Secondary active transporter
- The sodium-glucose linked transporter (Na+ / glucose co-transporters) SGLT-1
- SGLT1 means sodium glucose linked transporter 1
- This transporter transports them from the lumen to the mucosal cell
What is the transporter of fructose?
- Facilitated diffusion
- Not dependent on insulin
- GLUT5
- It is a uni-porter that is independent on sodium
- Transports fructose from the lumen into the mucosal cell
What is the transporter that transports monosaccharides from the mucosal cell into the portal circulation?
GLUT-2
What is the channel responsible for the secondary active transportation of the carbohydrates?
The sodium-potassium pump (Na+/K+), which establishes a sodium gradient by pumping it out of the cell using ATP
Summarize the digestion of carbohydrates
1) Mouth
- Starch
- Lactose
- Sucrose
- Cellulose
These carbohydrates are digested by a-amylase into:
- Starch dextrin
- Isomaltose
- Maltose
- Maltotriose
- Lactose
- Sucrose
- Cellulose
2) Stomcah
- In the stomach the Low pH denatures a-amylase and the pancreatic a-amylase takes its place converting the carbohydrates into:
- Isomaltose
- Maltose
- Maltotriose
- Lactose
- Sucrose
3) Mucosal cell membrane bound enzyme like:
- Isomaltase
- Lactase
- Sucrase
- Trehalase
- These enzymes will convert the carbohydrates to:
- Glucose
- Fructose
- Galactose
Then the carbohydrate will be taken to the portal circulation and to the liver
- Cellulose is excreted unchanged
What is meant by lactase deficiency?
- Lactase deficiency is when someone lacks the enzyme responsible for the cleavage of lactose
- Lactose will then get fermented by the gut bacteria into gas
- Lactose will also raise the osmotic pressure in the lumen resulting in a diarrhea
What are the types of lactase deficiency?
1) Primary lactase deficiency
2) Secondary lactase deficiency
What is meant by primary lactase deficiency?
- It is a congenital deficiency of lactase
- After the ingestion of lactose the affected child can experience stomach cramps, bloating, excess gas production and diarrhea, which might lead to failure to gain weight and failure to thrive
What is secondary lactase deficiency?
When someone has lactase deficiency due to an acquired cause like:
1) Gastroenteritis
2) Celiac disease
3) Crohn’s disease
4) Chemotherapy
5) Long course of antibiotics
How can we test for lactose intolerance?
1) Hydrogen breath test
The hydrogen breath test is used to diagnose either carbohydrate malabsorption (intolerance to sugars) or small intestine bacterial overgrowth
- This test measures the changes in the amount of hydrogen present in your breath after consuming hydrogen
What is meant by congenital sucrase-isomaltase deficiency?
- When a person congenitally lacks the enzymes sucrase and isomaltase
- Appears after an infant is weaned and starts consuming fruits, juices and grains
What are the different tests to diagnose congenital sucrase-isomaltase deficiency?
1) Small bowel biopsy
2) Sucrose intolerance hydrogen breath test
3) 13C sucrose breath test
4) 4-4-4 Ora; sucrose challenge test
What is the 13C sucrose breath test?
- A person consumes a sugary drink of sucrose that contains carbon-13
- If the person has little/no sucrase-isomaltase enzyme in their intestine there will be less 13CO2
- These monosaccharides and the one ingested in this test will ultimately lead to the production of 13C-labeled CO2, which is detected in the breath if sucrase is deficient then sucrose will remain undigested
What is the 4-4-4 Oral sucrose challenge test?
- It is a simple test that might indicate the presence of congenital sucrose-isomaltase deficiency
- A fasting patient should drink a well-dissolved solution of 4-tablespoon of table sugar in a 4-ounce glass
- Within 4 hours the patient will experience GI symptoms if the activity of the sucrase-isomaltase activity is diminished or absent
Summarize the digestion of proteins
1) Ingestion of dietary proteins
2) Pepsin and renin will then convert them to polypeptides via a condensation reaction between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another
3) They will then get converted to oligopeptide and amino acids
4) amino-peptidases will then convert them to amino acids which will be taken by the body to the liver and then the body
Where does the digestion of proteins start?
The stomach and then the intestinal lumen
What is the importance of gastric juice in the digestion of proteins?
1) HCl is used to convert the enzyme pepsinogen into pepsin
2) The gastric juice is also responsible for denaturing the proteins
What is the cell responsible for the secretion of pesin?
- The chief cells
- They secrete inactive pepsinogen, which gets activated by the hydrogen ion of the HCl
- Pepsin is an endopeptidase (which hydrolyzes peptide bonds)
- It is specific for tyrosine and glutamate, and its end results are polypeptides
How is the protein digested in the small intestine?
- The pancreas secretes Trypsin, chymotrypsin, and elastase into the small intestine
What is the function of trypsin?
It catalyzes the hydrolysis of lysine and arginine at the carboxy-terminal
What is the function of chymotrypsin?
It breaks the bond with the aromatic amino acid (tyrosine, tryptophan and phenylalanine)
What is the function of elastase?
It breaks the bond between the small neutral aliphatic amino acids from the ends of the peptides
Where is carboxy peptidase secreted and what is its function?
- It is secreted in the pancreatic juice, and its function is to release the amino acids from the carboxy terminal
What secretes aminopeptidase?
The intestinal mucosa
What is the function of aminopeptidases?
It releases amino acids from the amino-terminal
What is the function of dipeptidases and tripeptidases?
Found in the brush border of the intestinal mucosa they catalyze di and tri peptides
Aminopeptidases and carboxypeptidases are a type of which enzyme?
They are exopeptidases that hydrolyze peptide bonds at the terminal ends of a polypeptide chain
What is the function of endopeptidases?
- It includes enzymes like pepsin, trypsin and chymotrypsin
- They hydrolyze peptide bonds within the interior of a polypeptide chain
How are proteins absorbed?
1) Peptides are transported into intestinal cells via the Peptide Proton Cotransporter, which uses a H⁺ gradient to drive the uptake of dipeptides and tripeptides
2) Free amino acids are transported via sodium-amino acid cotransporters which rely on the sodium gradient created by the Na+/K+ ATPase pump
- Both are secondary active transport mechanisms (as they rely on the gradient of the ions)
- Both are symporters
What are the different diseases of protein malabsorption?
1) Hartnup’s disease
2) Iminoglycinuria
3) Cystinuria
What is Hartnup disease?
- It is a disease characterized by a defect in the transporter of non-polar amino acids that are not absorbed (tryptophan in particular)
- This disease occurs particularly in a defect in the apical brush border membranes of the small intestine and in the proximal tubules of the kidney
- Lack of reabsorption of tryptophan in the intestines or kidneys leads to its excessive loss in urine
- Since there is a defect in tryptophan serotonin, niacin, and melatonin will be affected
- The treatment is to administer niacin
What is iminoglycinuria?
- Due to the failure of absorption of PROLINE, HYDROXYPROLINE, AND GLYCINE from the kidney tubules
- It is an AUTOSOMAL recessive disorder of the renal tubular transport
- This results in excess urinary excretion of the three amino acids
What is cystinuria?
- A defect in the transport of cystine and other basic amino acids (COAL):
- Cystine
- Orthinine
- Arginine
- Lysine
- It is an inherited autosomal recessive disease
- It leads to the formation of cystine stones in the kidneys (cystine crystals), ureters, and bladder
What are the causes of cystinuria?
1) Mutation in the SLC3A1 and SLC7A9 genes responsible for the transport of amino acids
